
uniform mat4 ModelViewProjectionMatrix;
uniform float size;

in vec2 pos;
out vec2 radii;

void main()
{
  gl_Position = ModelViewProjectionMatrix * vec4(pos, 0.0, 1.0);
  gl_PointSize = size;

  // calculate concentric radii in pixels
  float radius = 0.5 * size;

  // start at the outside and progress toward the center
  radii[0] = radius;
  radii[1] = radius - 1.0;

  // convert to PointCoord units
  radii /= size;
}
